Many of the procedures of oilfield well servicing require that fluids and gases mixed with various chemicals and propants be pumped down the well tubing or casing under high pressures in the operations called acidizing and fracturing. This discussion will assume that a fluid is being pumped, and that the operation is that of acidizing. The wellhead fittings (spools, casing heads, tubing heads, valves, etc.) are normally rated for the pressure at which the well will be producing fluid while the pressures required for acidizing are several magnitudes higher than the producing pressure. It is thus required that some method of isolating the low pressure fittings from the high pressure fluid be used. Wells which are under pressure before the servicing is attempted are handled using a Well Tree Saver, Canada Pat. No. 1,094,945 or my invention, the Wellhead Isolation Tool, Canada Pat. No. 1,217,128, or some like apparatus. In the case of wells which are not under pressure and can have the array of wellhead valves and spools taken off prior to servicing, a different method of protecting the remaining low pressure fittings is utilized.
An example of the existing art is shown by the McEvoy PST adapter and packoff nipple shown in full section in FIG. 1 and, with a suggested but unreliable modification, in FIG. 2. It will be noted that there are a multitude of different wellhead arrangements where this adapter could be used which would include casing hangers along with the tubing hanger, but for this discussion, we will be limited to the simple casing head and tubing hanger.
Referring to FIG. 1, the casing head 1, is attached at its lower end to the casing 2, and holds in a cone shaped section, the tubing hanger 5, which holds the well tubing 3, in its lower end. The tubing head adapter 6, is attached to the casing head and has fitted and sealed inside it the packoff nipple 7, with its packing rings 8, the lower end of this packoff nipple being threaded into the tubing hanger. A cavity 10, is evident in the assembly. A high pressure valve 9, (or array of valves), completes the system. As can be seen, fluid passing through the open high pressure valve will pass through the packoff nipple, the tubing hanger and down the tubing. These are all high pressure rated fittings. The various seals and threaded fittings keep the high pressure fluid from contacting the casing head in the cavity 10. For this example, the casing head will be rated at 2000 psi. Under the new American Petroleum Institute (API) regulations, the tubing head adapter would be rated at 2000 psi at its lower end and 3000 psi at its upper end. (The regulations state that adapters are only allowed to raise the pressure rating by one rating division per adapter. The rating divisions are 2000 psi, 3000 psi, 5000 psi, 10000 psi, 15000 psi and 20000 psi.)
In order to obtain a higher pressure rating with this system, an assembly as shown in FIG. 2 would be required. The changeover spool 11, has been added, and the packoff nipple made longer to accommodate the added length. The cavity 10, is also larger. This assembly will allow a pressure of 5000 psi to be pumped through the array. To go up another pressure rating and pump at 10000 psi, another changeover spool would have to be added and the packoff nipple made longer. The disadvantages and objections to this type of stacking are:
a) Leakage past the seals on the packoff nipple will pressurize the cavity and damage the casing head. PA1 b) The height above the casing head of the high pressure valve causes problems in assembly and operation. PA1 c) The number of joints make leakage a distinct possibility. PA1 d) The time required to set up this array is expensive.